David Cameron has warned President Bashar al-Assad that he will face a day of reckoning for his savagery as Britain intensified the diplomatic pressure for a fresh UN security council resolution on Syria.

In some of his strongest criticism of Assad, the prime minister accused the Syrian leader of using Kofi Annan's peace plan to conduct rolling military operations against heavily populated areas.

"Far from fulfilling their commitments, the regime is cynically exploiting the window of diplomatic negotiations to crack down even harder on its own people," Cameron said in the Indonesian capital of Jakarta on the second leg of his Asian tour.

He called for the security council to throw all its weight behind the Annan plan and added: "Together we must ensure that there is a day of reckoning for Assad's crimes."

The prime minister spoke out on the day after the deadline for the Annan plan passed, amid estimates that 1,000 people have been killed in the last week in Syria. The peace plan devised by the former UN secretary general was approved by the UN and by Russia and China, which have repeatedly blocked security council resolutions on Syria.

Cameron, who spoke to Annan from Jakarta, attempted to intensify the pressure on Moscow and Beijing to drop their objections to a UN security council resolution. "It is clear that the world needs to redouble its efforts to stop the killing and intensify our support to those who oppose Assad's savagery," he said. "This is a decisive moment. The UN security council now has a clear responsibility to throw its full weight behind Kofi Annan's plan and to insist that it is implemented."

Downing Street sources said the prime minister believes China and Russia are looking increasingly out of step. "China and Russia have isolated themselves," one source said.

The prime minister outlined the scale of the violence in stark terms. Standing alongside the Indonesian president, Susilo Bambang Yudhoyono, at his palace, Cameron said: "We discussed the appalling bloodshed in Syria. We share deep concerns about what is happening.

"We should be very clear about the facts. Eleven days ago Assad made a commitment to UN envoy Kofi Annan to pull his forces out of urban areas and to stop using heavy weapons against his own people.

"This plan was backed by the UN, by the Arab League, by Russia, by China and the whole international community. The deadline for Assad to comply passed yesterday. But instead of stopping the killing, Kofi Annan now reports that Assad's troops are conducting rolling military operations in population centres supported by artillery fire. The rapid increase in refugees flowing into Turkey indicates a surge in violence.

"We estimate that 1,000 people have died in the last week, 300 over the Easter weekend. Far from fulfilling their commitments, the regime is cynically exploiting the window of diplomatic negotiations to crack down even harder on its own people.

"With increasing refugee flows across international borders, Assad's actions are now threatening regional peace and security. We want to achieve a negotiated end to the Syrian crisis and avoid full-scale civil war. But Assad seems bent on doing precisely the opposite."